GB2472758A

GB2472758A - Improved Insulator and Thermocouple

Info

Publication number: GB2472758A
Application number: GB0902732A
Authority: GB
Inventors: Paul Kevin Jackman; Gordon Whyte
Original assignee: MULTI LAB QUARTZ TEC Ltd
Current assignee: MULTI LAB QUARTZ TEC Ltd
Priority date: 2009-02-19
Filing date: 2009-02-19
Publication date: 2011-02-23
Also published as: GB0902732D0

Abstract

A thermocouple comprises a glass tube and an insulator rod which comprises an elongate element including a plurality of bores 6 formed therein. Adjacent bores lie substantially parallel to each other. The insulator rod is formed of quartz as a single unitary element. A single thermocouple wire 7 extends through each of the bores 6. The glass tube includes a bend and to insulate the thermocouple wires, small beads are cut from the insulator rod. The thermocouple may be adapted to measure temperatures at different positions in a furnace and this is achieved by locating thermocouple junctions 8 at different distances from the end of the tube. Openings 9 are cut in the insulator rod to reveal pairs of bores 7. At each opening 9 adjacent thermocouple wires are cut and joined to form junctions 8.

Description

Improved Insulator and Thermocouple

Field of the Invention

The present invention relates to insulators for thermocouples and to a multi-i2 thermocouples.

Background of the Invention

Thermocouples are used for the purpose of temperature measurement in many different applications. One type of thermocouple used to measure the temperature within furnaces of the type used in the micro-electronics industry comprises quartz (glass) tube within which are mounted pairs of platinum and rhodium wires. Where the wires of a pair meet and are joined is known as a junction. Clearly the wires of the pair must be kept apart other than at the junction. This is achieved by use of an insulator having a plurality of bores, each bore receiving a single wire. Where it is desirable to measure the temperature at a number of points across a furnace a multi-junction thermocouple may be used. For example, five pairs of wires providing five junctions may be inserted into a ten bore insulator. Each pair of wires is terminated at a different distance from the end of the thermocouple tube so that a temperature profile within the furnace may be measured.

Typically, the insulator has been made of alumina oxide. This material has good electrical insulation properties and is simple to form into a multi-bore rod for insertion into a quartz glass tube.

However, making the insulator of alumina oxide gives rise to a number of problems. First, the insulator expands upon being heated, which can cause breakages of the thermocouple wires. Second, the internal surface of the quartz tube may become damaged by the insulator.

Typically, insertion of the multi-bore rod into the quartz glass tube can scratch the inner surface of the quartz glass tube. Still further, the quartz glass tube generally includes a bend.

The thermocouple wires must be insulated around this bend. This is achieved by cutting small lengths (for example 3 mm lengths) of the insulator to surround the thermocouple wires around the bend. The edges of these small lengths can scratch the inner surface of the quartz tube.

Where the surface of the quartz tube is damaged the alumina oxide can react with the quartz of the tube causing de-vitrification thereof.

Hence, thermocouples comprising a quartz glass outer tube, an alumina oxide insulating rod and thermocouple wires have a relatively short service life.

It would therefore be desirable to provide an improved thermocouple.

Were the insulator to be formed of the same material as the quartz tube some of the problems associated with the alumina oxide would be overcome.

It is known to use quartz glass to protect a thermocouple wire. Published United Kingdom Patent Application No 999,798 describes a thermocouple in which the thermocouple wire at the junction is protected by a close-fitting thin-walled sheath formed of electrically insulating, heat refractory material, such as high temperature glass, silica or quartz. In this thermocouple the outer tube is formed of cardboard, and within the cardboard tube the thermocouple wire is insulated by twin bore ceramic tube or a block of high temperature cement. A similar thermocouple is described in US 5,975,755.

A multi-bore thermocouple where wires are protected by quartz glass tubes is described in US 5,772,324. In this multi-bore thermocouple an outer tube is provided which surrounds a reinforcing member. Insulating quartz glass tubes are situated in the space formed between the outer tube and the reinforcing member. The quartz glass tubes are protected by a graphite coating.

Whilst it is known to insulate thermocouple wire using glass, the prior art thermocouples are complex and therefore costly to construct and have a different configuration to the thermocouples commonly used in furnaces used in the micro-electronics industry.

It would be desirable to provide a thermocouple having a similar construction to those using alumina oxide insulators.

Summary of the Invention

According to the invention there is provided an insulator rod consisting of an elongate element including a plurality of bores formed therein, wherein adjacent bores lie substantially parallel to each other and are separated by a small distance, wherein the insulator rod is formed of quartz as a single unitary element.

Preferably each bore is configured to receive a thermocouple wire.

According to another aspect of the invention there is provided a thermocouple comprising a quartz tube at least one pair of thermocouple wires joined together at a junction and an insulator rod consisting of an elongate element including a plurality of bores formed therein, wherein adjacent bores lie substantially parallel to each other and are separated by a small distance, wherein the insulator rod is formed of quartz as a single unitary element. located within and extending along the quartz tube. Advantageously, each bore is configured to receive a thermocouple wire.

The quartz of the insulator rod may be opaque or transparent.

The quartz tube may include a bend, and the thermocouple may include a plurality of short sections of insulator rod, arranged in the quartz tube around the bend.

According to a further aspect of the invention there is provided an insulator bead comprising a section of insulator divided from an insulator rod consisting of an elongate element including a plurality of bores formed therein, wherein adjacent bores lie substantially parallel to each other and are separated by a small distance, wherein the insulator rod is formed of quartz as a single unitary element..

The insulating rod of the invention allows a thermocouple to be manufactured using the same techniques as are used where the insulating rod is formed of alumina oxide, yet provides a thermocouple having a longer service life potential. By making the insulating rod of the same material as the tube, i.e. quartz, there should be no significant thermal expansion between the rod and the tube, which will reduce breakages of thermocouple wires. Further, the inner surface of the thermocouple tube is less likely to damaged by the outer surface of the quartz insulator, again because the materials are the same. Still further, in the event that there is some damage to the inner surface of the tube, there is a lower chance of re-crystallisation occurring, again, because the insulator and the tube are of the same material.

Whilst the use of quartz as an insulator in a thermocouple is known, it is not known to provide an insulator made solely of quartz. Making the insulator solely of quartz provides a significant advantage in potential service life of a thermocouple.

Brief Description of the Drawings

In the drawings, which illustrate preferred embodiments of the invention, and are by

way of example:

Figure 1 is a schematic representation of a thermocouple according to the invention; Figure 2 is an end view of an insulator of the thermocouple illustrated in Figure 1; Figure 3 is a side view of the insulator illustrated in Figure 2; Figure 4 is a schematic representation of the insulator illustrated in Figures 2 and 3; and Figure 5 is a schematic representation of an insulator housing five thermocouples.

Detailed Description of the Preferred Embodiments

Referring now to Figures 1 to 5, there is shown a thermocouple 1 comprising a glass tube 2 having a closed end 3, which in use protrudes into an environment such as a furnace and an end 4 which in use is located outside the monitored environment. The thermocouple further comprises an insulator rod 5 which includes a plurality of bores 6 (ten as can be seen from Figures 2 and 4). A single thermocouple wire 7 extends through each of the bores 6.

Wires 7 of adjacent bores are joined together at a junction 8.

The tube 1 includes a bend 10. The thermocouple wires 7 must be insulated around the bend 10. This is achieved by cutting small pieces of insulator rod 5, the small pieces being referred to as beads 11. The beads 11 can orient themselves to negotiate their way around the bend 10. For an insulator rod 5 having an external diameter of 6 mm, the beads would for example be between 3 mm and 18 mm long (of course different lengths of bead may be selected). In the drawing the beads illustrated have the same length. However, the beads of different lengths may be used to negotiate a bend, with the shorter beads being used at the apex of the bend and longer beads (possibly of progressively increasing length) being used to either side of the apex of the shorter beads.

The thermocouple 1 is adapted to measure temperatures at different positions in a furnace. This is achieved by locating thermocouple junctions 8 at different distances from the end 3 of the tube 2. As can be seen from Figure 5, openings 9 are cut in the insulator rod 5 to reveal pairs of bores 7. At each opening 9 adjacent thermocouple wires are cut and joined to form a junction 8.

The insulator rod 5 is formed of quartz glass.

In the illustrated example the insulator rod 5 has a diameter of 5.50mm and the bores 6 each have a diameter of 0.80mm and are centred on a diameter of 3.80mm. The gap between adjacent bores is approximately 0.4mm. The insulator rod 5 must therefore be manufactured to a very high degree of accuracy, since it is the glass material between adjacent bores that provides electrical insulation between thermocouple wires in those bores from each other.

Whilst the problems associated with alumina oxide insulators have been known for some time, and whilst it has been known for some time that quartz glass can be used as an insulator in a thermocouple, there is no example in the prior art of a multi-bore insulating rod formed of quartz glass. As mentioned above, the insulator rod 5 must be manufactured to a very high degree of accuracy.

Claims

Claims 1. An insulator rod comprising an elongate element including a plurality of bores formed therein, wherein adjacent bores lie substantially parallel to each other and are separated by a small distance, wherein the insulator rod is formed of quartz as a single unitary element.
2. An insulator according to Claim 1, wherein each bore is configured to receive a thermocouple wire.
3. A thermocouple comprising a quartz tube at least one pair of thermocouple wires joined together at a junction and an insulator rod as claimed in Claim 1 or 2 located within and extending along the quartz tube.
4. A thermocouple according to Claim 3, wherein the quartz tube includes a bend, and wherein the thermocouple includes a plurality of short sections of insulator rod, arranged in the quartz tube around the bend.
5. An insulator bead comprising a section of insulator divided from an insulator rod according to Claim 1 or 2.
6. An insulator substantially as shown in, and as described with reference to, the drawings.
7. A thermocouple substantially as shown in, and as described with reference to, the drawings.